Pneumatic tires for automobiles these days are required to have a wide variety of properties, including fuel economy as well as abrasion resistance, wet-skid performance and the like. Various methods have been devised to improve these properties. For example, silica-containing rubber compositions are used as fuel-efficient formulations. They further contain silane coupling agents because silica has lower affinity for rubber than carbon black does.
Though the use of bis(3-triethoxysilylpropyl)disulfide or bis(3-triethoxysilylpropyl)tetrasulfide, which are widely used as silane coupling agents, increases the dispersibility of silica and thus improves mechanical properties, the silane coupling agent needs to be added in a large amount, which leads to a great increase in cost. In addition, even the addition of a large amount of the silane coupling agent results in insufficient dispersibility in some cases. Furthermore, kneading at a high temperature for sufficient reaction between silica and the silane coupling agent tends to cause gelation of rubber or compound scorch.
Silane coupling agents containing a mercapto group, on the other hand, are known for being highly reactive and effective in increasing the dispersibility of silica. With these silane coupling agents, however, scorch time tends to be shortened so that compound scorch can easily occur in a final kneading step or an extrusion step. Thus, they are difficult to use particularly in SBR-containing systems, which are more likely to scorch than low-polarity rubbers such as NB and BR.
Patent Literature 1 discloses a rubber composition for tires which contains silica and which can enhance wet grip performance without deterioration in rolling resistance and abrasion resistance. This rubber composition, however, still has room for improvement in terms of achieving a balanced improvement in these properties while maintaining good processability.